As the Fossil Museum/Fossilary entry states, Guanlong, which Guan is based on, is a close relative of the Tyrannosaurus. This most likely inspired the move Great Unmasking, which transforms Guan into T-Rex.

However, if Guan is Super Revived (Silver Fossil), its increased stat total will not be added to the stats of the transformed T-Rex, nor will it become T-Rex Lord if the Guan has a Miraculous Fossil Rock appiled.